Division of Biology, Imperial College London, Silwood Park, Ascot, Berkshire, SL5 7PY, UK.
Glob Chang Biol. 2012 Sep;18(9):2824-37. doi: 10.1111/j.1365-2486.2012.02732.x. Epub 2012 Jun 27.
Increases in the emissions and associated atmospheric deposition of nitrogen (N) have the potential to cause significant changes to the structure and function of N-limited ecosystems. Here, we present the results of a long-term (13 year) experiment assessing the impacts of N addition (30 kg ha(-1) yr(-1) ) on a UK lowland heathland under a wide range of environmental conditions, including the occurrence of prolonged natural drought episodes and a severe summer fire. Our findings indicate that elevated N deposition results in large, persistent effects on Calluna growth, phenology and chemistry, severe suppression of understorey lichen flora and changes in soil biogeochemistry. Growing season rainfall was found to be a strong driver of inter-annual variation in Calluna growth and, although interactions between N and rainfall for shoot growth were not significant until the later phase of the experiment, N addition exacerbated the extent of drought injury to Calluna shoots following naturally occurring droughts in 2003 and 2009. Following a severe wildfire at the experimental site in 2006, heathland regeneration dynamics were significantly affected by N, with a greater abundance of pioneering moss species and suppression of the lichen flora in plots receiving N additions. Significant interactions between climate and N were also apparent post fire, with the characteristic stimulation in Calluna growth in +N plots suppressed during dry years. Carbon (C) and N budgets demonstrate large increases in both above- and below-ground stocks of these elements in N-treated plots prior to the fire, despite higher levels of soil microbial activity and organic matter turnover. Although much of the organic material was removed during the fire, pre-existing treatment differences were still evident following the burn. Post fire accumulation of below-ground C and N stocks was increased rapidly in N-treated plots, highlighting the role of N deposition in ecosystem C sequestration.
氮（N）排放和相关的大气沉积增加有可能导致氮限制生态系统的结构和功能发生重大变化。在这里，我们展示了一项长期（13 年）实验的结果，该实验评估了在广泛的环境条件下，包括长时间自然干旱事件和严重夏季火灾的情况下，N 添加（30 kg ha(-1) yr(-1)）对英国低地石南荒地的影响。我们的研究结果表明，氮沉降的增加对石南的生长、物候和化学性质有很大的持久影响，对林下地衣植物群有严重的抑制作用，并改变了土壤生物地球化学。发现生长季节的降雨量是石南生长年际变化的主要驱动因素，尽管氮和降雨对新梢生长的相互作用直到实验的后期阶段才变得显著，但氮添加加剧了 2003 年和 2009 年自然干旱后石南新梢的干旱损伤程度。在实验地点发生严重野火后，石南荒地的再生动态受到氮的显著影响，在接受氮添加的斑块中，先锋苔藓物种的丰度增加，地衣植物群受到抑制。火灾后还明显存在气候和氮之间的相互作用，在+N 斑块中，在干旱年份，石南的生长特征刺激受到抑制。碳（C）和 N 预算表明，在火灾前，N 处理的斑块中地上和地下元素的储量都大幅增加，尽管土壤微生物活性和有机物质周转率较高。尽管在火灾中大部分有机物质都被清除，但在燃烧后仍能明显看出预先存在的处理差异。火灾后，N 处理的斑块中地下 C 和 N 储量迅速积累，突出了氮沉积在生态系统 C 固存中的作用。
